In space-based synthetic aperture radars of conventional design, the antenna must be large enough to suppress ambiguities in the radar image. For example, the antenna size for the C-band Canadian Radarsat satellite is 15 m, as described in P. J. Wood, "Design of Slotted Waveguide Arrays for the Radarsat SAR Antenna", Symposium on Antenna Technology and Applied Electromagnetics (ANTEM), Ottawa, August 1994, pp 188-195.
The antenna size may be reduced while still using conventional SAR processing algorithms. This implies compromising the swath width (which determines the amount of mapping data which can be generated in a given time), or the image quality, as described in L. M. Martins-Camelo et al, "Systems Considerations for Active SAR Antennae", Symposium on Antenna Technology and Applied Electromagnetics (ANTEM), Montreal, August 1996, pp 65-68.
FIG. 1 illustrates the principle of operation of a satellite which uses SAR. Two distinct types of imaging process are involved, one for the along-track (or `azimuth`), the other for cross-track (or `elevation`) case.
As shown, `elevation` image is mapped across the coverage swath. This swath lies totally to one side of the satellite track, and may typically be 100 Km. wide. The antenna beam 1 transmitted from antenna 3 is usually several degrees wide in the elevation plane, and illuminates all of the coverage swath. However, different image pixels across the swath can be distinguished by the different time delays of the corresponding received radar pulses. Radar signals which are reflected back from the outer edge of the swath (the edge furthest from the satellite track) travel a longer distance and therefore take a longer time to return to the satellite. The elevation resolution of the ground image is determined by the smallest detectable difference in timing. This resolution is of the order of 20 m. in the case of the Canadian Radarsat satellite.
In the azimuth plane, the antenna beam is usually much narrower. In the Radarsat case, it is 0.2 degrees wide. On the ground, the beam stretches some 3 Km. across the along-track dimension. The 3 km. wide spot moves with the satellite, to achieve along-track mapping. Because the satellite is moving rapidly around its orbit, the radar returns are subject to the classical `Doppler effect`. Thus, for a return from a part of the ground which is ahead of the spacecraft in relation to its motion, the spacecraft is moving towards the ground. As a result, the frequency of the radar reflection is increased. Conversely, for a return from a part of the ground which is behind the spacecraft, the spacecraft is moving away from the ground, and the frequency of the radar reflection is decreased. The SAR processor uses small changes in frequency of the received signal to distinguish between radar targets which are close together in the along-track dimension. Thus, it is able to subdivide the 3 Km. along-track spot into many azimuth pixels. In the Radarsat case, an azimuth resolution of the order of 20 m. is achieved in this way.
The minimum size for a SAR antenna has been largely dictated by the need to avoid `ambiguities`. When no ambiguities are present, a large signal level at a particular pixel of a SAR-generated image implies that there is a strong reflection from the radar target at the one unique point on the illuminated swath which corresponds to that pixel. However, when ambiguities are present, two or more different target positions exist which could give rise to a signal at the same image pixel. For a SAR on a spacecraft, these different positions typically lie some hundreds of kilometers apart on the earth's surface.
A key system parameter for a SAR is the PRF (Pulse Repetition Frequency). To avoid azimuth ambiguities, the received signals must be sampled at at least the rate implied by the classical Fourier sampling theorem (two samples per cycle), bearing in mind the bandwidth of the Doppler spectrum. As already described, the bandwidth of the Doppler spectrum is essentially proportional to the along-track length of the ground footprint of the antenna beam.
A large PRF implies liberal sampling. The beam footprint can then be large in the azimuth plane, while still maintaining ambiguity-free azimuth image processing. Under these circumstances, the azimuth dimension of the antenna can be relatively small.
The pulse period (time interval between pulses) may be calculated as the reciprocal of the PRF. In the elevation plane, ambiguities tend to occur when, in addition to the desired radar targets, other target positions exist which generate reflected pulses which arrive an integral number of pulse periods before or after the reflection from the desired target. Thus, a large PRF will tend to cause range ambiguities for the elevation plane image processing.
In the past, range ambiguities have been overcome by reducing the swath width of the radar beam. This however implies that the antenna must generate a narrow elevation beam, and hence its elevation aperture dimension must be large.
It has been determined that, to avoid major ambiguity problems, the area of the SAR antenna aperture must exceed a particular value. In principle, this value depends upon certain parameters of the spacecraft orbit, specifically spacecraft platform velocity and orbit height. In practice, however, these latter parameters end to be fairly similar for all spacecraft SAR applications.
In principle, the width of a SAR swath can ideally be such that the change in delay of the received signals from the near side of the swath to the far side of the swath approaches one pulse period. In practice, spacecraft SAR systems are often designed for a swath width of about one half of the ideal case. Under these conditions, the prior art SAR antenna must always have an aperture area of at least 10 square meters at C-band, or 40 square meters at L-band. Even at C-band the size of the SAR antenna becomes one of the principal factors determining the size of the satellite bus. At L-band or lower frequencies, the size of the antenna tends to be a major disincentive, discouraging implementations at these frequencies.
It will thus be seen from the above that there is a fundamental constraint on aperture area for a SAR antenna, and that this constraint comes about via a combination of limitations set by the SAR azimuth processing algorithms, and the SAR elevation processing algorithms. For prior art SAR's, the properties of the antenna beam are used to suppress both azimuth and elevation ambiguities.
Nominally, a spacecraft SAR needs to use a radar pulse whose effective time duration is very short, in order to make it possible to detect very small arrival time differences, and create an image which has a high resolution in the elevation plane. However, as shown in FIG. 2, it is standard practice to transmit a sequence of swept-frequency pulses 5 generated in a pulse generator 7, called chirp pulses.
The radar receiver incorporates a `matched filter` 9 to compress the received pulses 11. For the type of implementation shown in FIG. 2 in which an `upwards chirp` is used (the frequency increases during the pulse), the matched filter 9 delays the components of the pulse which are at higher frequencies, so that all frequency components add coherently at its output. Conversely, for a `downwards chirp` case, the matched filter will delay the components of the pulse which are at lower frequencies. Thus, the matched filter transforms the very low-level, wide, swept-frequency pulse 11 into a single, very narrow pulse 13 which has a much larger amplitude.
In a prior art SAR design, the parameters of the expanded swept frequency pulse, and the matched filter which compresses it, have been fixed quantities.
In general, when pulse compression is used in a radar, a relatively long pulse is transmitted, the length of the pulse ensuring that the radar echo contains enough energy to be easily detectable by the receiver. In order that the echo can be timed very accurately, even after the pulse has perhaps been distorted as a result of reflection from a radar target, the long pulse is configured in some special way, so that it is possible to distinguish each individual small part of it from all the other small parts. One way of doing this is to change the frequency during the pulse (the `chirp pulse`), so that each part of the pulse has a different frequency. Another (coded sub-pulses) is to split the pulse into a long series of many short sub-pulses. The sub-pulses form a sequential code of nominally `0` and one values. For example, such a sequence might be EQU 0110100
although in practice a real sequence would be very much longer. The code is carefully configured so as to avoid embedding any repeated sequence of `0`s and `1`s in it.
In the basic `coded sub-pulse` scheme, the subpulses amplitude-modulate the radar frequency carrier, essentially turning it on and off. In another scheme, the carrier is transmitted at all times, but the sub-pulses change the phase of the radar-frequency signal, a `0` sub-pulse giving one phase, and a `1` sub-pulse giving a different phase. Finally, in yet another version (some times referred to as pulse compression via phase codes), there are in general several different phase values, not just two.
All these schemes have in common that a specific `code` is transmitted: for example an upwards frequency sweep of 20 MHz extent for a chirp pulse, or a specific sequence of `1`s and `0`s for the coded sub-pulse approach. There is then always a matched filter. The filter is designed to look for precisely the code that has been transmitted, and to extract its all-important timing information.
U.S. Pat. No. 5,608,404 to Burns et al describes a system in which either one or more than one of center frequency, starting phase, and chirp rate transmitted pulses are varied on a pulse-to-pulse basis in response to radar motion. It is thus restricted to a real time system. Pulse-to-pulse variation of these specific parameters is effected between one azimuth sub-aperture and the next. Thus the chirp rate actually remains constant for a group of pulses which are within one subaperture.